In the present state of the art known electromagnetic transducers comprise a permanent magnet connected to a fixed structure and magnetically interacting with a rotating toothed wheel fixed to the member the speed of rotation of which, with respect to the fixed structure, is to be measured. As is well known, variation in the magnetic flux (d.phi.) caused as the teeth pass in proximity to the permanent magnet, induces in a coil wound about a part of the magnet an electromotive force proportional to d.phi./dt, where t is the time. In known transducers a coil is wound on the permanent magnet or on an iron core forming part of a magnetic circuit including the magnet. Normally the current induced in the coil is sinusoidal and, by means of a simple frequency-voltage converter circuit a signal the amplitude of which is proportional to the speed of rotation of the toothed wheel is obtained.
In general the permanent magnets used in known devices described above are, effectively, magnetic flux generators in that their poles are well spaced and do not form part of a closed magnetic circuit, thus creating very little leakage flux. Therefore small pole faces are adequate as the magnetic flux across the faces is small. In fact the permanent magnets are operated in the region of their hysteresis cycle close to the remanence point; bearing in mind the shape of the magnetisation curve in the fourth quadrant of the well known hysteresis diagram which results when H is plotted against B (H magnetic field, B magnetic induction) it follows from this that in order to obtain adequate variation of B to create sufficient flux change to excite the coil in order to generate a detectable voltage signal, it is necessary to cause a considerable variation of H. In practical terms this means that it is necessary to maintain a very small and constant distance during rotation between the teeth of the wheel and the magnet. This requires an almost impossibly perfect adjustment and frequent inspection of the parts subject to play and/or misalignment due to wear of mountings and bearings. In fact even the smallest variation of the relative positions of the movable toothed wheel and the fixed magnetic circuit can induce a variation of magnetic flux sufficient to cause an induced electromotive force which would noticeably affect the output signal, hence introducing a considerable error into the operation of the transducer.